Optical agents currently play a central role in a large number of in vivo, in vitro and ex vivo clinical procedures including important diagnostic and therapeutic procedures. Photodiagnostic and phototherapeutic agents, for example, include a class of molecules capable of absorbing, emitting, and/or scattering electromagnetic radiation applied to a biological material, particularly in the visible and/or near infrared regions of the electromagnetic spectrum. This property of optical agents is used in a range of biomedical applications for visualizing, imaging or otherwise characterizing biological materials and/or achieving a desired therapeutic outcome. Recent developments in targeted administration and delivery of optical agents, and advanced systems and methods for applying and detecting electromagnetic radiation in biological environments has considerably expanded the applicability and effectiveness of optical agents for clinical applications.
Important applications of optical agents that absorb and/or emit in the visible and/or near-infrared (NIR) region of the electromagnetic spectrum include their use in biomedical imaging and visualization. For example, compounds absorbing and/or emitting light in these regions of the electromagnetic spectrum currently are useful for optical tomography, optoacoustic tomography, optical coherence tomography, confocal scanning laser tomography, optical coherence tomography, and fluorescence endoscopy; techniques which have emerged as essential molecular imaging techniques for imaging and visualizing biological processes at the organ, cellular and subcellular (e.g., molecular) levels. Biomedical images are generated, for example, by detecting electromagnetic radiation, nuclear radiation, acoustic waves, electrical fields, and/or magnetic fields transmitted, emitted and/or scattered by components of a biological sample. Modulation of the energy or intensity of the applied radiation yields patterns of transmitted, scattered and/or emitted radiation, acoustic waves, electrical fields or magnetic fields that contain useful anatomical, physiological, and/or biochemical information. A number of applications of biomedical imaging have matured into robust, widely used clinical techniques including planar projection and tomographic X-ray imaging, magnetic resonance imaging, ultrasound imaging, and gamma ray imaging.
Established optical imaging and visualization techniques are based on monitoring spatial variations in a variety of optical parameters including the intensities, polarization states, and frequencies of transmitted, reflected, and emitted electromagnetic radiation. Given that many biological materials of interest are incompatible with ultraviolet light, research is currently directed to developing and enhancing imaging techniques using visible and near infrared (NIR) radiation (from about 400 nm to about 900 nm). In particular, NIR light (700 nm to 900 nm) is useful for visualizing and imaging deeper regions than visible light because electromagnetic radiation of this wavelength range is capable of substantial penetration (e.g., up to four centimeters) in a range of biological media. Optical imaging and visualization using optical agents has potential to provide a less invasive and safer imaging technology, as compared to X-ray, and other widely used nuclear medicine technologies. Applications of optical imaging for diagnosis and monitoring of the onset, progression and treatment of various disease conditions, including cancer, are well established. (See, e.g., D. A. Benaron and D. K. Stevenson, Optical time-of-flight and absorbance imaging of biologic media, Science, 1993, 259, pp. 1463-1466; R. F. Potter (Series Editor), Medical optical tomography: functional imaging and monitoring, SPIE Optical Engineering Press, Bellingham, 1993; G. J. Tearney et al., In vivo endoscopic optical biopsy with optical coherence tomography, Science, 1997, 276, pp. 2037-2039; B. J. Tromberg et al., Non-invasive measurements of breast tissue optical properties using frequency-domain photon migration, Phil. Trans. Royal Society London B, 1997, 352, pp. 661-668; S. Fantini et al., Assessment of the size, position, and optical properties of breast tumors in vivo by noninvasive optical methods, Appl. Opt., 1998, 37, pp. 1982-1989; A. Pelegrin et al., Photoimmunodiagnosis with antibody-fluorescein conjugates: in vitro and in vivo preclinical studies, J. Cell Pharmacol., 1992, 3, pp. 141-145).
Optical agents for in vivo and in vitro biomedical imaging, anatomical visualization and monitoring organ function are described in International Patent Publication WO2008/108941; U.S. Pat. Nos. 5,672,333; 5,698,397; 6,167,297; 6,228,344; 6,748,259; 6,838,074; 7,011,817; 7,128,896, and 7,201,892. In this context, optical imaging agents are commonly used for enhancing signal-to-noise and resolution of optical images and extending these techniques to a wider range of biological settings and media. In addition, use of optical imaging agents having specific molecular recognition and/or tissue targeting functionality has also been demonstrated as effective for identifying, differentiating and characterizing discrete components of a biological sample at the organ, tissue, cellular, and molecular levels. Further, optical agents have been developed as tracers for real time monitoring of physiological function in a patient, including fluorescence-based monitoring of renal function. (See International Patent Publication PCT/US2007/0149478). Given their recognized utility, considerable research continues to be directed toward developing improved optical agents for biomedical imaging and visualization.
In addition to their important role in biomedical imaging and visualization, optical agents capable of absorption in the visible and NIR regions have also been extensively developed for clinical applications for phototherapy. The benefits of phototherapy using optical agents are widely acknowledged as this technique has the potential to provide efficacy comparable to radiotherapy, while entirely avoiding exposure of non-target organs and tissue to harmful ionizing radiation. Photodynamic therapy (PDT), in particular, has been used effectively for localized superficial or endoluminal malignant and premalignant conditions. The clinical efficacy of PDT has also been demonstrated for the treatment of various other diseases, injuries, and disorders, including cardiovascular disorders such as atherosclerosis and vascular restenosis, inflammatory diseases, ophthalmic diseases and dermatological diseases. Visudyne and Photofrin, for example, are two optical agents that have been developed for the treatment of macular degeneration of the eye and for ablation of several types of tumors, respectively. (See, e.g., Schmidt-Drfurth, U.; Bringruber, R.; Hasan, T. Phototherapy in ocular vascular disease. IEEE Journal of Selected Topics in Quantum Electronics 1996, 2, 988-996; Mlkvy, P.; Messmann, H.; Regula, J.; Conio, M.; Pauer, M.; Millson, C. E.; MacRobert, A. J.; Brown, S. G. Phototherapy for gastrointestinal tumors using three photosensitizers—ALA induced PPIX, Photofrin, and MTHPC. A pilot study. Neoplasma 1998, 45, 157-161; Grosjean, P.; Wagieres, G.; Fontolliet, C.; Van Den Bergh, H.; Monnier, P. Clinical phototherapy for superficial cancer in the esophagus and the bronchi: 514 nm compared with 630 nm light irradiation after sensitization with Photofrin II. British Journal of Cancer 1998, 77, 1989-1955; Mitton, D.; Ackroyd, R. Phototherapy of Barrett's oesophagus and oesophageal carcinoma—how I do it. Photodiagnostics and Phototherapy 2006, 3, 96-98; and Li, L.; Luo, R.; Liao, W.; Zhang, M.; Luo, Y.; Miao, J. Clinical study of photofrin phototherapy for the treatment of relapse nasopharyngeal carcinoma. Photodiagnostics and Phototherapy 2006, 3, 266-271; See, Zheng Huang “A Review of Progress in Clinical Photodynamic Therapy”, Technol Cancer Res Treat. 2005 June; 4(3): 283-293; “Photodiagnosis and Photodynamic Therapy”, Brown S, Brown E A, Walker I. The present and future role of photodynamic therapy in cancer treatment. Lancet Oncol. 2004; 5:497-508; Triesscheijn M, Baas P, Schellens J H M. “Photodynamic Therapy in Oncology”; The Oncologist. 2006; 11:1034-1044; and Dougherty T J, Gomer C J, Henderson B W, Jori G, Kessel D, Korbelik M, Moan J, Peng Q. Photodynamic Therapy. J. Natl. Cancer Inst. 1998; 90:899-905).
Phototherapy is carried out by administration and delivery of a photosensitizer to a therapeutic target tissue (e.g., tumor, lesion, organ, etc.) followed by photoactivation of the photosensitizer by exposure to applied electromagnetic radiation. Phototherapeutic procedures require photosensitizers that are relatively chemically inert, and become activated only upon irradiation with light of an appropriate wavelength. Selective tissue injury can be induced with light when photosensitizers bind to the target tissues, either directly or through attachment to a bioactive carrier or targeting moiety. Photosensitizers essentially operate via two different pathways, classified as Types 1 and 2. A primary distinction between these classes of photosensitizers is that the Type 1 process operates via direct energy or electron transfer from the photosensitizer to the cellular components thereby inducing cell death, whereas the Type 2 process involves first the conversion of singlet oxygen from the triplet oxygen found in the cellular environment followed by either direct reaction of singlet oxygen with the cellular components or further generating secondary reactive species (e.g. peroxides, hydroxyl radical, etc.) which will induce cell death.
The Type 1 mechanism proceeds via a multistep process involving activation of the photosensitizer by absorption of electromagnetic radiation followed by direct interaction of the activated photosensitizer, or reactive intermediates derived from the photosensitizer, with the target tissue, for example via energy transfer, electron transfer or reaction with reactive species (e.g., radicals, ions, nitrene, carbene, etc.) resulting in tissue damage. The Type 1 mechanism can be schematically represented by the following sequence of reactions:

wherein hv indicates applied electromagnetic radiation and (PHOTOSENSITIZER)* indicates excited state of the photosensitizer. The Type 2 mechanism proceeds via a three-step process involving activation of the photosensitizer by absorption of electromagnetic radiation followed by energy transfer from the activated photosensitizer to oxygen molecules in the environment of the target tissue. This energy transfer process generates excited state oxygen (1O2) which subsequently interacts with the target tissue so as to cause tissue damage. The Type 2 mechanism can be schematically represented by the following sequence of reactions:
wherein hv indicates applied electromagnetic radiation, (PHOTOSENSITIZER)* indicates photoactivated photosensitizer, 3O2 is ground state triplet oxygen, and 1O2 is excited state singlet oxygen.
The biological basis of tissue injury brought about by tumor phototherapeutic agents has been the subject of intensive study. Various biochemical mechanisms for tissue damage have been postulated, which include the following: a) cancer cells up-regulate the expression of low density lipoprotein (LDL) receptors, and phototherapy (PDT) agents bind to LDL and albumin selectively; (b) porphyrin-like substances are selectively taken up by proliferative neovasculature; (c) tumors often contain increased number of lipid bodies and are thus able to bind to hydrophobic photosensitizers; (d) a combination of “leaky” tumor vasculature and reduced lymphatic drainage causes porphyrin accumulation referred to as “EPR” (enhanced permeability and retention) effect; (e) tumor cells may have increased capabilities for phagocytosis or pinocytosis of porphyrin aggregates; (f) tumor associated macrophages may be largely responsible for the concentration of photosensitizers in tumors; and (g) cancer cells may undergo apoptosis induced by photosensitizers. Among these mechanisms, (f) and (g) are the most general and, of these two alternatives, there is a general consensus that (f) is the most likely mechanism by which the phototherapeutic effect of porphyrin-like compounds is induced.
Much of the research in the past several decades has focused on developing phototherapeutic agents based on the Type 2 (PDT) mechanism. Surprisingly, there has been considerably less attention devoted to Type 1 phototherapeutic agents despite the fact that there are numerous classes of compounds that could potentially be useful for phototherapy that function via this mechanism. Unlike Type 2, the Type 1 process does not require oxygen; and hence Type 1 photosensitizers are expected to be potentially more effective than Type 2 photosensitizers under hypoxic environments typically found in solid tumors. Second, the Type 1 mechanism involves two steps (photoexcitation and direct energy transfer), whereas the Type 2 mechanism involves three steps (photoexcitation, singlet oxygen generation, and energy transfer). Further, studies have recently shown that production of high levels of reactive oxygen species can induce an anti-inflammatory response, which may result in blood vessels to become more “leaky,” thereby increasing the risk of metastasis (Chen, B.; Pogue, B.; Luna, J. M.; Hardman, R. L.; Hoopes, P. J.; Hasan, T. Tumor vascular permeabilization by vascular-targeting photosensitization: effects, mechanism, and therapeutic implications. Clinical Cancer Research 2006, 12(3, Pt. 1), 917-923). Targeted Type 1 photosensitizers, by their very nature, are not expected to produce reactive oxygen species; rather, the reactive species produced by these photosensitizers will immediately react with the cellular component at the binding site and trigger cell death. Type 2 phototherapeutic agents, however, do have certain advantages over Type 1 agents. For example, Type 2 agents can potentially be catalytic, i.e., the Type 2 photosensitizer is regenerated once the energy transfer to the oxygen has taken place. In contrast, Type 1 process would generally be expected to require stoichiometric amounts of the photosensitizer in some clinical settings. Table I provides a summary of the attributes of Type 1 and Type 2 phototherapeutic agents. Given these attributes, it is clear that development of safe and effective Type 1 phototherapeutic agents would be useful to complement the existing therapeutic approaches provided by Type 2 agents, and to enhance the therapeutic portfolio available for clinicians.
TABLE 1Comparison between Type 1 and Type2 processes for phototherapy.TYPE 1 PROCESSTYPE 2 PROCESSTwo-step process.Three-step process.Not well explored.Very well studied.Light of any wavelengthRequires red light for optimalcan be used.performance.Does not require oxygen.Requires oxygen.Large classes of compounds.Limited classes of compounds.Stoichiometric.Potentially catalytic.Intramolecular energy transfer toIntermolecular energy transfer togenerate reactive species.generate reactive oxygen species.No products in the market.Two products are in use.
Specific optical, chemical and pharmacokinetic properties of optical agents are necessary for their effective use in Type 1 and Type 2 phototherapeutic applications. For example, optical agents for these applications preferably have strong absorption in the visible or NIR regions, and also exhibit low systemic toxicity, low mutagenicity, and rapid clearance from the blood stream. These optical agents must also be compatible with effective administration and delivery to the target tissue, for example by having reasonable solubilities and a low tendency for aggregation in solution. Upon excitation by absorption of visible and NIR electromagnetic radiation, optical agents for Type 1 and 2 phototherapy preferably provide large yields of singlet oxygen (Type 2) or other reactive species, such as free radicals or ions, capable of causing local tissue damage. Both Type 1 and Type 2 photosensitizers typically undergo photoactivation followed by intersystem crossing to their lowest triplet excited state, and therefore, a relatively long triplet lifetime is usually beneficial for providing effective tissue damage. Other useful properties of optical agents for these applications include chemical inertness and stability, insensitivity of optical properties to changes in pH, and compatibility with conjugation to ligands providing targeted delivery via molecular recognition functionality. Multifunctional optical agents have also been developed for phototherapy that are capable of providing both imaging and visual functionality upon excitation at a first range of wavelengths and phototherapeutic functionality upon excitation at a second range of wavelength. (See, U.S. Pat. No. 7,235,685 and International Patent Publication WO 2007/106436).
Optical agents for phototherapeutic applications preferably exhibit a high degree of selectivity for the target tissue. Selectivity provided by optical agents facilitates effective delivery to a target tissue of interest and provides a means of differentiating different tissue classes during therapy. Selective tissue injury can be induced with light when photosensitizers bind to the target tissues either directly, as in the case of Photofrin, or through attachment to a bioactive carrier, or through in situ biochemical synthesis of the photosensitizer in localized area, as in the case of 2-aminolevulinic acid, which is an intermediate in the biosynthesis of porphyrin. Previous studies have shown that certain dyes localize in tumors and serve as a powerful probe for the detection and treatment of small cancers. (D. A. Belinier et al., Murine pharmacokinetics and antitumor efficacy of the photodynamic sensitizer 2[I-hexyloxyethyl]-2-devinyl pyropheophorbide-a, J. Photochem. Photobiol., 1993, 20, pp. 55-61; G. A. Wagnieres et al., In vivo fluorescence spectroscopy and imaging for oncological applications, Photochem. Photobiol., 1998, 68, pp. 603-632; J. S. Reynolds et al., Imaging of spontaneous canine mammary tumors using fluorescent contrast agents, Photochem. Photobiol., 1999, 70, pp. 87-94). It is generally recognized, however, that many of these dyes do not localize preferentially in malignant tissues. A number of strategies have been developed for imparting selectivity and/or targeting functionality by incorporation of a molecular recognition component in the optical agent. For example, targeting of fluorescent dyes to tumors has been demonstrated by us and others using dye conjugates with antibodies and peptides for diagnostic imaging of tumors. (See, Achilefu et al., Novel receptor-targeted fluorescent contrast agents for in vivo imaging of tumors, Investigative Radiology, 2000, 35, pp. 479-485; Ballou et al., Tumor labeling in vivo using cyanine conjugated monoclonal antibodies, Cancer Immunology and Immunotherapy, 1995, 41, pp. 257-263; and Licha et al., New contrast agent for optical imaging: acid cleavable conjugates of cyanine dyes with biomolecules, in Biomedical Imaging: Reporters, Dyes and Instrumentation, Proceedings of SPIE, 1999, 3600, pp. 29-35). Therefore, receptor-target mediated phototherapy agents provide a promising pathway for achieving site selective activation at various target tissues.
For both photodiagnostic and phototherapeutic applications, optical agents preferably exhibit a high degree of selectivity for the target tissue. Selectivity provided by optical agents facilitates effective delivery to a target tissue of interest and provides a means of differentiating different tissue classes during imaging, visualization and therapy. There is a considerable need for developing optical agents for biomedical applications that have high absorption/emission properties in the visible and NIR regions, high photostability, insensitivity to pH, and wavelength tunability, as well as selectivity for the target tissue.